In a case of a cured material for an optical member, for example, a prism sheet for enhancing luminance, it is possible to improve front luminance of backlight by increasing the refractive index of a cured resin layer, and in a case of Fresnel lens, it is possible to make the lens pattern shallower as a resin layer has a higher refractive index. Thus, from the aspect that the productivity can be improved by facilitating release from a mold, the high refractive index of the cured resin has been desired.
As a process for producing such a cured material for an optical member, particularly, an optical sheet, a process of utilizing a pressing process, a cutting process, an extrusion process or the like to obtain a prism sheet for liquid crystal improvement, Fresnel lens for Projection TV, Lenticular lens, and the like is proposed (for example, refer to PTL 1).
However, any of the production processes has low productivity, and currently, a process for forming an optical resin layer such as a prism layer and a lens layer on a transparent sheet substrate such as a transparent plastic sheet by using an active energy ray-curable composition has been utilized.
In order to impart high hardness and scratch resistance to such a curable composition, the use of a particle dispersion of inorganic oxides such as silica and zirconia in combination is disclosed (for example, refer to PTL 2).
Further, as a process for obtaining the zirconia particle dispersion, in order to provide a transparent dispersion, a process for performing dispersion with media of 0.05 mm or more using an acetylacetone type dispersion aid is described (for example, refer to PTL 3). It is described that a zirconia particle dispersion having a small dispersed particle size is obtained according to the aforementioned process. However, in a case where the acetyl acetone type dispersion aid is used, there is a disadvantage that deterioration and coloration due to heat and light tend to occur.
A process for producing dispersion of fine inorganic particles, which includes supplying a silane coupling agent last among the respective materials, that is, a zirconium oxide nanoparticle, a dispersant, a dispersion medium, and the silane coupling agent to be supplied to the media type wet dispersing machine, has been proposed (for example, refer to PTL 4). According to the producing process of PTL 4, it is possible to produce a cured material for an optical member which has a high transparency, is stable against heat, and is also excellent in yellowing resistance. However, this process has a problem in that overdispersion occurs when mild dispersion conditions of using media having a small particle diameter of 30 μm or less and lowering the solid concentration at the time of dispersing are not set. The media having a small particle size is very expensive and for which available dispersing machines are limited. In addition, when the media having a small particle size is used and the solid concentration is set to be lowered, the dispersion efficiency is deteriorated, and the process time is significantly prolonged. Another process is to disperse using a large amount of dispersants, but it also includes the problem in that the refractive index of the obtained cured composition is decreased.
A process for mixing and dispersing all of the respective materials, that is, an aggregate of metal oxide nanoparticles, a dispersant, metal alkoxide, and a solvent to be supplied to the dispersing machine before being disintegrated has been proposed (for example, refer to PTL 5). PTL 5 discloses that the amount of dispersant can be reduced so as to solve the problems such as bleed-out and decrease in hardness by the aforementioned production process; however, this process has problems in that the dispersion efficiency is deteriorated, and the process time is significantly prolonged, and the production efficiency is lowered.